The present invention relates to a combustion chamber structure for an internal combustion engine in which a combustion chamber is defined by a lower surface of an inner wall of a cylinder head which confronts a cylinder bore and an upper surface of a piston which slidably fits in the cylinder bore and in which a predetermined gap is formed between the lower surface of the inner wall of the cylinder head and the upper surface of the piston when the piston reaches the top dead center.
As approaches to prevention of the occurrence of knocks by sufficiently stirring air-fuel mixture within combustion chambers of an internal combustion engine, there are proposed an approach in which swirls are generated in the combustion chambers and an approach in which squishes are generated in the combustion chambers. The swirl is a spiraling flow of air-fuel mixture around the piston axis and is generated by the shape and position of the inlet port or by an inlet valve having a shroud. The squish is a radial flow of air-fuel mixture around the piston axis and is generated based on increased and decreased gaps formed between the piston and the cylinder head as the piston fluctuates.
The Unexamined Japanese Patent Application Publication No.2002-89266 describes a combustion chamber structure for an internal combustion engine for preventing the occurrence of knocks by generating a squish. In this combustion chamber structure, squishes directed towards a central portion of the combustion chamber are generated by flat slopes provided on the upper surface of the piston at diametrical ends thereof which intersect with an axis of a piston pin at right angles, and circumferential squishes are generated by conical slopes provided continuously on circumferential ends of the flat slopes. Then, the circumferential squishes are caused to collide with each other from opposite directions at the diametrical ends which follow the axis of the piston pin so as to deflect the circumferential squishes towards the central portion of the combustion chamber, whereby the squishes are spouted from the four circumferential portions of the combustion chamber towards the central portion thereof.
With the combustion chamber structure according to the related art, however, since the squishes are spouted from the four circumferential portions of the combustion chamber towards the central portion thereof, each squish becomes weak and hence it is difficult to sweep completely air-fuel mixture existing in an outer circumferential portion of the combustion chamber to the central portion thereof.